Many additive systems for improving wet-end drainage and fines retention have been disclosed in the prior art. The systems include employing polymers, combinations of polymers and polymers combined with colloidal silica. The latter systems are among the most efficient now in use, but there is a continuing need to provide additives with reduced cost and improved performance.
This invention employs, as a retention and drainage aid, polysilicate microgels formed by the partial gelation of an alkali metal silicate or a polysilicate such as sodium polysilicate, having in its most common form one part Na.sub.2 O to 3.3 parts SiO.sub.2 by weight. The microgels, which are referred to as "active" silica in contrast to commercial colloidal silica, consist of aggregates of very small, e.g., 1 nm, particles arranged into three dimensional networks and chains. They typically have a very high surface area, usually greater than 1000 square meters per gram (m.sup.2 /g) and are used in the papermaking process in conjunction with cationic polymers derived from natural or synthetic sources.
Polysilicate microgels can be formed most readily by the addition of an initiator to a solution of sodium polysilicate. The initiator starts the process of gelation which, if allowed to proceed to completion, would result in the total solidification of the solution. The gel solidification time, that is the length of time for total solidification to occur once initiated, the can range from seconds to months and depends on a variety of factors including pH, silica concentration, temperature and the presence of neutral salts. For commercial applications, short gel solidification times are preferred. Once initiated, gelation is allowed to proceed for about 5% to 95% of the gel solidification time before being stopped by diluting the polysilicate solution, preferably to about 1 weight percent (wt%) SiO.sub.2 or less.
The solution of polysilicate microgels so formed has been found to constitute an excellent retention and drainage aid when combined with a water soluble cationic polymer, preferably cationic starch, cationic guar or cationic polyacrylamide.